Dark Matter
Dark matter accounts for 80% of known gravitational mass in the universe, but can't be seen directly. Background Gravity is a force that is weaker the further apart the two bodies are. So for a planet orbiting a star like the sun the planet experiences a much stronger pull towards the sun if it is closer. This is best imagined as like having a bowling ball in the centre of a trampoline and then rolling marbles around it. If you roll a marble very close to the ball then it will likely roll towards it very quickly, due to the large slope. If you roll it at the edge of the trampoline then it will only be drawn to the centre weakly at first. Orbits are attained if you roll the marble at the perfect speed so that it constantly falls towards the ball, but has so much speed that it keeps going around it, and not so much speed that it simply flies off the trampoline completely (escape velocity). Because gravity is stronger closer to the sun, planets that orbit close to the sun have to maintain a faster velocity. While outer planets like Neptune and former planet Pluto can orbit more slowly and still resist being drawn in. Hence, there is a trend in solar systems for the orbital speed to decrease with the orbital radius. Taking this out to the galactic scale, all of the stars (including our Sun) are in orbit around a giant cluster of stars and a super-massive black hole in the centre of the milky way. While the milky way is much more densely populated that our solar system, it was still initially thought that similar logic would apply. There was an expectation that stars would orbit at slower speeds the further they were from the centre of the galaxyhttps://en.wikipedia.org/wiki/Milky_Way#Galactic_rotation. However, it was instead found that stars in our galaxy tend to orbit at a fairly constant speed independent of their distance from the galactic centre. This fundamentally defied the model that the centre of the galaxy is the 'bowling ball' and the trillions of stars are many tiny marbles affected by the gravitational slope it causes. Instead, the slope seems to be fairly constant, implying that the milky way's mass is not concentrated just in the centre, but is more uniformly distributed. This defies our visual evidence of star densities and implies that there is a lot of mass in our galaxy that is surrounding areas with lower densities of stars. Increasing the slope in regions where there previously was thought to be no reason for gravitational mass. Properties Mostly undetermined, see http://en.wikipedia.org/wiki/Dark_matter. http://www.astronomy.com/news/2018/05/the-case-against-dark-matter https://www.reddit.com/r/space/comments/8hr7hw/emergent_gravity_seeks_to_replace_the_need_for/ "Physicists are painfully aware of the fact that spiral galaxies are spinning faster than they should be, given the amount of matter — and therefore, gravity — they contain. At the speed that some of them are spinning, current theory says that the stars, planets, dust, and other matter should be flung off into space. Because they are not, physicists have hypothesized that “dark matter” we cannot see or otherwise detect is causing the extra gravitational pull, keeping these galaxies together. This matter is said to account for about 25 percent of the universe, but Verlinde believes that there may be another answer that can account for the deviations between the expected and observed rotation curves. “What is observed is that the deviations that we see in the rotational curves of galaxies, which is just derived by looking at the matter that we see, always seems to occur at one particular acceleration,” he says. That particular acceleration happens to play an important role in the relationship between a galaxy’s distance and the speed with which it’s moving away from our own, which is governed by the expansion of the universe, known as Hubble’s Law. A 2017 paper by Alexandre Chaloum Elbeze in the Journal of Modern Physics outlines how the expansion rate of the universe, or H0, is linked through a new parameter, which he calls E0, is linked to the rotation curves of galaxies measured by astronomers." Dark Energy New insights on dark energy - PhysOrg "For several decades cosmologies have successfully used a relativistic equation with dark matter and dark energy to explain increasingly precise observations about the cosmic microwave background, the cosmological distribution of galaxies, and other large-scale cosmic features. But as the observations have improved, some apparent discrepancies have emerged. One of the most notable is the age of the universe: there is an almost 10% difference between measurements inferred from the Planck satellite data and those from so-called Baryon Acoustic Oscillation experiments. The former relies on far-infrared and submillimeter measurements of the cosmic microwave background and the latter on spatial distribution of visible galaxies. CfA astronomer Daniel Eisenstein was a member of a large consortium of scientists who suggest that most of the difference between these two methods, which sample different components of the cosmic fabric, could be reconciled if the dark energy were not constant in time." Doing without dark energy: Mathematicians propose alternative explanation for cosmic acceleration - PhysOrg "Three mathematicians have a different explanation for the accelerating expansion of the universe that does without theories of "dark energy." Einstein's original equations for General Relativity actually predict cosmic acceleration due to an "instability," they argue in paper published recently in Proceedings of the Royal Society A. About 20 years ago, astronomers made a startling discovery: Not only is the universe expanding—as had been known for decades—but the expansion is speeding up. To explain this, cosmologists have invoked a mysterious force called "dark energy" that serves to push space apart." "Three mathematicians have a different explanation for the accelerating expansion of the universe that does without theories of "dark energy." Einstein's original equations for General Relativity actually predict cosmic acceleration due to an "instability," they argue in paper published recently in Proceedings of the Royal Society A. About 20 years ago, astronomers made a startling discovery: Not only is the universe expanding—as had been known for decades—but the expansion is speeding up. To explain this, cosmologists have invoked a mysterious force called "dark energy" that serves to push space apart." Links Dark matter and black holes. (potential connections to Hydrinos) :"In 1933, Caltech astronomer Fritz Zwicky noticed that the galaxies within the Coma cluster were orbiting one another too quickly. Much too quickly.... :Based on the visible luminosity of the Coma galaxies, they should have been orbiting one another at about 80 km/s. Instead, they were moving over 1–2,000 km/s. He was led to speculate: ::“dark matter is present in much greater amount than luminous matter.”" :"Vera Florence Cooper Rubin (/ˈruːbɪn/; July 23, 1928 – December 25, 2016) was an American astronomer who pioneered work on galaxy rotation rates. She uncovered the discrepancy between the predicted angular motion of galaxies and the observed motion, by studying galactic rotation curves." References Category:Astronomy Category:Astrophysics Category:Physics Category:Mass Category:Matter